Distinguishing between buried semiconductor/metal contacts and hybrid semiconductor/metal/liquid contacts at n-GaAs/KOH-Se-/2-(aq) junctions

The current-voltage properties of n-GaAs photoanodes have been evaluated in KOH-Se^-/2-(aq), CH₃CN-Fc^+/0, and CH₃CN-MV^2+/+ solutions. Chemisorption of transition-metal ions (Rh^III, Co^III, Ru^III, Os^III) onto n-GaAs has been shown previously to effect improved photoanode behavior for n-GaAs/KOH-Se^-/2-(aq) contacts, but it is not clear whether the chemisorbed metal forms a buried semiconductor/metal junction or results in a hybrid semiconductor/metal/liquid contact. After chemisorption of transition-metal ions, n-GaAs photoanodes displayed different open circuit voltages in contact with each electrolyte solution investigated. The role of the chemisorbed metal in the n-GaAs/M/KOH-Se^-/2-(aq) system is, therefore, best described as catalyzing interfacial charge transfer at the semiconductor/liquid interface, as opposed to establishing a semiconductor/metal or semiconductor/ insulator/metal contact that is exposed to, but not influenced by, the electrolyte solution.